Method of fixing carbon dioxide comprising a step of recycling the used extraction solvent

ABSTRACT

A method of fixing carbon dioxide is provided. After metal ion components are extracted from natural mineral or steel slag through acid treatment, carbon dioxide is injected to fix carbon dioxide by carbonating the same. Since the procedure of pH adjustment is unnecessary, the reaction is carried out effectively, and a continuous process is enabled. Further, since the used extraction solvent is recycled, the cost of fixing carbon dioxide is reduced. The disclosed method of fixing carbon dioxide enables effective removal of carbon dioxide produced from the steelmaking industry, thereby significantly reducing greenhouse gas emission and allowing recycling of the conventionally discarded steel slag.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2010-0117700, filed on Nov. 24, 2010, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND

(a) Technical Field

The present invention relates to a method of fixing carbon dioxide. Moreparticularly, the present invention relates to a method for fixingcarbon dioxide utilizing natural mineral or steel slag, thereby reducingemission of carbon dioxide into the atmosphere.

(b) Background Art

Global carbon dioxide emissions have been rapidly increasing due toincreased fossil fuel consumption. Carbon dioxide emissions have beenrecently recognized as the main cause of global warming, and thus,countries around the world are making efforts to provide appropriateregulations to reduce such emissions.

The reduction in carbon dioxide emissions can be accomplished byreducing the use of fossil fuel itself, or by a method wherein thecarbon dioxide produced by the use of fossil fuel is separated,collected and fixed. The separated and collected carbon dioxide can, forexample, be used as a source for methanol synthesis. The separated andcollected carbon dioxide can also be fixed by dumping it into the oceanor by using carbonate minerals.

According to one method, carbon dioxide emitted from plants is fixedusing carbonate minerals. In particular, the carbon dioxide is reactedwith alkaline substances (e.g. CaO, MgO, K₂O, Na₂O, etc.) existing inminerals to produce carbonates (e.g. CaCO₃, MgCO₃, Na₂CO₃, K₂CO₃, etc.).

Slags produced from steelmaking processes include molten ironpretreatment slag, converter slag, stainless steel slag, electricfurnace slag, and the like. These steel slags are mostly buried, with alimited amount being utilized as cement or aggregates for road orbuilding construction. Various other methods for utilizing the discardedsteel slag are desired and have been proposed, particularly as itbecomes more difficult to find landfills in which to bury the slag.

Korean Patent Application Publication No. 2002-0050429 proposes a methodof fixing carbon dioxide on the surface of steel slag for use in harborconstruction or artificial fish banks for fish. Korean PatentApplication Publication No. 2006-0023206 proposes fixing carbon dioxideon the surface of slag containing a certain level of water by reactingit with carbon dioxide. However, these two methods require an excessivereaction time since the efficiency of the reaction between carbondioxide and slag is very low. Thus, the methods are economicallyimpractical.

Korean Patent No. 0891551 (Application No. 2008-0025573) proposes amethod of solidifying carbon dioxide by carbonating alkaline componentsextracted from steel slag with gaseous carbon dioxide through either apressurized or a normal-pressure hydrothermal reaction. However, thehydrothermal reaction consumes a great deal of energy, and it is unclearhow the carbon dioxide can be fixed following the carbonation of thealkaline components.

Korean Patent No. 0801542 (Application No. 2006-0105753) proposes amethod of fixing carbon dioxide using natural mineral talc. However,according to this method the particles must be 125 μm or less, thusrequiring excessive energy for the pulverization of mineral. Further,after the alkaline components are treated with a weak acid solvent, suchas acetic acid, for extraction, the pH is increased to fix the carbondioxide through carbonation. This increase in pH causes some metal ions(e.g., Ca²⁺) to react with hydroxide ions (OH⁻) to form milky lime(Ca(OH)₂), resulting in suspension. Thus, an additional precipitation orfiltration procedure is required. Since the precipitated carbonate issuspended in the solution, the separation is difficult and it isdifficult to establish an industrially applicable continuous process.

SUMMARY

The present invention provides to a method for effectively fixing carbondioxide that overcomes the problems with conventional methods. Thepresent method is applicable to actual processes, whereby carbon dioxidecan be fixed via liquid-phase reaction. The present method furtherprovides improved reaction efficiency, at normal temperature undernormal pressure, thus reducing energy consumption. Further, according tothe present methods, it is unnecessary to adjust pH for carbon dioxidefixation, thus preventing the formation of suspended milky lime andallowing for a continuous process.

According to further embodiments, the present invention provides amethod capable of lowering the cost of fixing carbon dioxide byrecycling the used extraction solution after the separation ofcarbonates.

In a general aspect, the present invention provides a method of fixingcarbon dioxide comprising: (a) treating natural mineral or steel slagwith an acid to extract metal ion components; (b) injecting carbondioxide into an extraction solution containing the metal ion componentsobtained in step (a) to carbonate the same; (c) transferring a solutionin which carbonates resulting from step (b) are dissolved to a dissolvedcarbonate storage tank and storing the same; (d) transferring the storeddissolved carbonates to a carbonate separation/purification tank andadjusting pH to 7 or above to separate the carbonates; and (e) treatingthe solution from which the carbonates have been separated with an acid,and recycling the treated solution as the extraction solvent (acid) instep (a).

The above and other aspects and features of the present invention willbe described infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will now be described in detail with reference to certainexemplary embodiments thereof illustrated in the accompanying drawingswhich are given hereinbelow by way of illustration only, and thus arenot limitative of the disclosure, and wherein:

FIG. 1 shows an existing process of fixing carbon dioxide using slag;and

FIG. 2 shows a method of fixing carbon dioxide using natural mineral orsteel slag according to the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of thedisclosure. The specific design features of the disclosure as disclosedherein, including, for example, specific dimensions, orientations,locations and shapes, will be determined in part by the particularintended application and use environment.

DETAILED DESCRIPTION

Hereinafter, reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the disclosure will bedescribed in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit thedisclosure to those exemplary embodiments. On the contrary, thedisclosure is intended to cover not only the exemplary embodiments, butalso various alternatives, modifications, equivalents and otherembodiments, which may be included within the spirit and scope of thedisclosure as defined by the appended claims.

The present invention provides a method of fixing carbon dioxidecomprising:

(a) treating natural mineral or steel slag with an acid to extract metalion components;

(b) injecting carbon dioxide into an extraction solution containing themetal ion components obtained in step (a) to carbonate the same;

(c) transferring the solution of step (b), in which carbonates resultingfrom step (b) are dissolved, to a dissolved carbonate storage tank andstoring the same;

(d) transferring the stored dissolved carbonates to a carbonateseparation/purification tank and adjusting pH to 7 or above to separatethe carbonates; and

(e) treating the solution from which the carbonates have been separated(in from step (d)) with an acid, and recycling the acid treated solutionfor use as the extraction solvent (acid) in step (a).

In step (a), the natural mineral is not particularly limited. Forexample, peridotite, basalt, talc, serpentinite, wollastonite, etc.,which contain a large amount of metal oxides, such as calcium oxideand/or magnesium oxide, may be used. The contents of calcium oxide andmagnesium oxide in serpentinite and wollastonite are shown, for example,in Table 1.

In step (a), the steel slag may be selected from, but is not limited to,slag, electric furnace slag or converter slag produced from, forexample, an ironworks during blast furnace, converter or oxygen blowingprocesses. Currently, steel slag is mostly utilized as cement oraggregates for road or building construction. The chemical compositionof various steel slag is also shown in Table 1.

TABLE 1 CaO (wt %) MgO (wt %) Natural mineral Serpentinite 0 40Wollastonite 48 0 Steel slag Blast furnace slag 41 10 Converter slag 462 Electric furnace slag 20 5 Ladle furnace slag 54 10

Since the natural mineral and the steel slag contain a large amount ofmetal oxides, including calcium oxide and magnesium oxide, they exhibithigh basicity. As such, upon acid treatment, the metal components of themetal oxides are released into the solution, thus exhibiting alkalineproperty. Generally, these metal ion components are mostly Ca² and Mg²⁺,and the remainder may be, for example, K⁺, Na⁺, etc.

In accordance with some embodiments, the acid treatment may be performedat about pH 3-5. To provide the desired pH, an acidic substance such asacetic acid, sodium acetate, hydrochloric acid, etc., may be used at asuitable concentration.

Upon acid treatment, the solution containing the metal ions, e.g., Ca²⁺,Mg²⁺, etc., is provided with an acidic pH.

According to the conventional methods for fixing by carbonation, the pHof the extraction solution is adjusted to about 12, followed byinjection of carbon dioxide to form a carbonate precipitate. During thisprocedure, some of the dissolved metal ions (e.g., Ca²⁺, Mg²⁺, etc.)react with hydroxide ions (OH⁻) to form milky lime, thus resulting insuspension. Unless the suspending solid is effectively separated, itbecomes difficult to fix carbon dioxide through a continuous process. Asa result, treatment of carbon dioxide on a large scale is verydifficult, if not impossible.

In accordance with the present invention, the procedure of adjusting thepH to about 12 is omitted and, thus, this limitation is overcome. Inparticular, by omitting the pH adjustment procedure, the presentinvention avoids the formation of milky lime and, consequently, theformation of suspending solids. Thus, the present invention allows for acontinuous process, and treatment of carbon dioxide on a large scale ispossible. According to the present invention, the procedure foradjusting pH to an alkaline range (pH 7 or above) for separation of thecarbonates is carried out after the CO2 reaction with metal ioncomponents is completed. Since the metal ions have participated inreactions to form the carbonates, there is very little possibility thatthey will form suspending solids through reaction with hydroxide ions.The solution in which the carbonates (i.e. the carbonates resulting fromthe step (b) carbon dioxide fixing) are dissolved is then transferred toa dissolved carbonate storage tank and for storage. Subsequently, thestored dissolved carbonate solution is transferred to a carbonateseparation/purification tank, and the pH of the solution is adjusted to7 or above to separate the carbonates. Consequently, the procedures ofcarbonation and carbonate separation can be performed continuously.

In embodiments of the present invention, the carbonation in step (b) maybe performed at normal temperature under normal pressure. As referred toherein, “normal temperature” and “normal pressure” mean that noadditional heat or pressure needs to be applied. For example, thetemperature may be about 0-40° C., and in accordance with someembodiments about 10-25° C., and the pressure may be about 0.1-5 atm,and in accordance with some embodiments about 0.5-2 atm. In accordancewith the present invention, the carbonation can be completed within 5minutes without requiring an energy-consuming hydrothermal reaction.According to some embodiments, the rate of injection of the carbondioxide in step (b) can be controlled so as to provide a desired timefor completion of carbonation. In particular, according to variousembodiments, the carbonation reaction of step (b) can be completedwithin 5 minutes when the injection rate of carbon dioxide is about 2L/min. Further, since the release of the metal ion components in step(a) can be accomplished within 2 hours, the overall process can becompleted in about 2 hours, making it a very economical process. Thus,carbon dioxide can be effectively fixed using natural mineral or steelslag (e.g., slag, electric furnace slag or converter slag produced froman ironworks during blast furnace, converter or oxygen blowingprocesses), thereby remarkably reducing the emission of greenhouse gasand allowing the utilization of the previously discarded steel slag.

The carbon dioxide gas treated by the method according to the presentinvention may be any carbon dioxide gas, including those produced asby-products from industrial furnaces such as blast furnaces, limeburning furnaces, coking furnaces, etc., sintering or hot-rollingprocesses, power generation, waste heat boilers, and the like.Accordingly, if an ironworks is equipped with a facility capable offixing carbon dioxide generated in the steelmaking process, the steelslag produced during the process may be utilized to fix the globalwarming-causing gas on site. Through this, environmental regulations canbe met, and by-products such as metal oxides can be utilized as a newsource of revenue, rather than waste.

According to the present invention, the solution with the carbonatesseparated in step (d) is not discarded but can be recycled as theextraction solvent (also referred to as “acid”) in step (a). Since thesolution resulting from step (d) has a pH of 7 or above, it is firsttreated with an acid to adjust pH to 3-5 for subsequent use as theextraction solvent in step (a). The acid treatment may be carried outusing one or more acid(s) preferably selected from a group consisting offormic acid, acetic acid, propionic acid, phosphoric acid, phosphorousacid, citric acid and oxalic acid. The recycled extraction solvent canbe mixed with a fresh extraction solvent. and the mixture supplied tothe extracting device (as shown, for example, in FIG. 2). By recyclingthe extraction solution, the cost related to preparation, purificationand disposal of the extraction solution is reduced, and the overall costof carbon dioxide fixing can be reduced significantly.

In accordance with the present invention, a method is provided wherebycarbon dioxide is fixed by carbonation without pH adjustment, andwhereby high-purity carbonate can be prepared in about 2 hours. Further,since a continuous process is enabled by the present method, treatmentof carbon dioxide in large scale is possible. In addition, since theextraction solution can be recycled, the cost related to preparation,purification and disposal of the extraction solution can be reduced andthe overall cost of carbon dioxide fixing can be reduced significantly.

The method of fixing carbon dioxide according to the present invention,whereby metal ions are extracted through acid treatment of naturalmineral or steel slag, is much more effective than the existingsolid-gas reaction methods. Since the reaction is carried out at normaltemperature under normal pressure, without requiring an energy-intensiveprocedure such as hydrothermal synthesis, energy consumption can bereduced. Further, since the pH adjustment for fixing carbon dioxide isomitted, formation of suspended milky lime can be prevented, and acontinuous process is enabled. Further, since the extraction solutioncan be recycled, the cost related to preparation, purification anddisposal of the extraction solution can be reduced and the overall costof carbon dioxide fixing can be reduced significantly.

Since the method of fixing carbon dioxide according to the presentinvention allows reduction of carbon dioxide emissions using steel slag,which is currently used only for valueless applications, it can be aneffective measure to cope with the greenhouse gas reduction requirementimposed on steelmaking companies, while allowing for recycling of thediscarded steel slag as carbonate.

In addition, it is noted that the cost related to preparation,purification and disposal of the extraction solution used in a carbondioxide fixing process accounts for more than 50% of the total cost offixing carbon dioxide. Thus, according to the present invention, theoverall cost of carbon dioxide fixing is reduced by reducing the costrelated to preparation, purification and disposal of the extractionsolution. Further, the net amount of fixed carbon dioxide can beenhanced as the amount of the used extraction solution is decreased.

The present invention has been described in detail with reference tospecific embodiments thereof. However, it will be appreciated by thoseskilled in the art that various changes and modifications may be made inthese embodiments without departing from the principles and spirit ofthe disclosure, the scope of which is defined in the appended claims andtheir equivalents.

1. A method of fixing carbon dioxide comprising the steps of: (a)treating natural mineral or steel slag with an acid to extract metal ioncomponents, thereby providing an extraction solution containing themetal ion components; (b) injecting carbon dioxide into the extractionsolution containing the resulting metal ion components to carbonate thesame, thereby providing a solution in which resulting carbonates aredissolved; (c) transferring the solution in which the resultingcarbonates are dissolved to a dissolved carbonate storage tank; (d)transferring the solution from the dissolved carbonate storage tank to acarbonate separation/purification tank, and adjusting pH to 7 or aboveto separate the carbonates; (e) treating the solution from which thecarbonates have been separated with an acid; and (f) recycling thetreated solution to step (a) as an extraction acid.
 2. The method offixing carbon dioxide according to claim 1, wherein said treating withthe acid in step (e) is performed at about pH 3-5.
 3. The method offixing carbon dioxide according to claim 1, wherein said carbonating isperformed at normal temperature under normal pressure.
 4. The method offixing carbon dioxide according to claim 1, wherein step (e) comprisesadjusting pH to about 3-5 using one or more acid(s) selected from agroup consisting of formic acid, acetic acid, propionic acid, phosphoricacid, phosphorous acid, citric acid and oxalic acid.